Malda, West bengal Jul 9, 2021 (Issuewire.com) - Concrete is a construction material composed of cement, fine aggregates (sand), and coarse aggregates mixed with water which hardens with time. Portland cement is the commonly used type of cement for the production of concrete. Concrete technology deals with the study of the properties of concrete and its practical applications.
In building construction, concrete is used for the construction of foundations, columns, beams, slabs, and other load-bearing elements.
There are different types of binding material is used other than cement such as lime for lime concrete and bitumen for asphalt concrete which is used for road construction.
Various types of cement are used for concrete works which have different properties and applications. Some of the types of cement are Portland Pozzolana Cement (PPC), rapid hardening cement, Sulphate resistant cement, etc.
Materials are mixed in specific proportions to obtain the required strength. The strength of mix is specified as M5, M10, M15, M20, M25, M30, etc, where M signifies Mix and 5, 10, 15, etc. as their strength in kN/m2. In the United States, concrete strength is specified in PSI which is Pounds per Square Inch.
Water cement ratio plays an important role which influences various properties such as workability, strength, and durability. An adequate water-cement ratio is required for the production of workable concrete.
When water is mixed with materials, cement reacts with water and a hydration reaction starts. This reaction helps ingredients to form a hard matrix that binds the materials together into a durable stone-like material.
Concrete can be cast in any shape. Since it is a plastic material in a fresh state, various shapes and sizes of forms or formworks are used to provide different shapes such as rectangular, circular, etc.
Various structural members such as beams, slabs, footings, columns, lintels, etc. are constructed with concrete.
ACI 318 Building code requirements for structural concrete and ACI 301 Specifications for Structural Concrete are used in the United States as standard code of practice for concrete construction.
There are different types of admixtures that are used to provide certain properties. Admixtures or additives such as pozzolans or superplasticizers are included in the mixture to improve the physical properties of the wet mix or the finished material.
Various types of concrete are manufactured these days for the construction of buildings and structures. These have special properties and features which improve the quality of construction as per requirement.
Components of Concrete
Components of concrete are cement, sand, aggregates, and water. A mixture of Portland cement and water is called a paste. So, concrete can be called a mixture of paste, sand, and aggregates. Sometimes rocks are used instead of aggregates.
The cement paste coats the surface of the fine and coarse aggregates when mixed thoroughly and binds them. Soon after mixing the components, hydration reaction starts which provides strength and rock-solid concrete is obtained.
What is the Grade of Concrete?
The grade of concrete denotes the strength required for construction. For example, the M30 grade signifies that the compressive strength required for construction is 30MPa. The first letter in grade “M” is the mix and 30 is the required strength in MPa.
Based on various lab tests, the grade of concrete is presented in Mix Proportions. For example, for M30 grade, the mix proportion can be 1:1:2, where 1 is the ratio of cement, 1 is the ratio of sand, and 2 is the ratio of coarse aggregate based on volume or weight of materials.
The strength is measured with concrete cubes or cylinders by civil engineers at the construction site. Cube or cylinders are made during casting of the structural member and after hardening, it is cured for 28 days. Then compressive strength test is conducted to find the strength.
Regular grades of concrete are M15, M20, M25, etc. For plain cement concrete works, generally, M15 is used. For reinforced concrete construction minimum M20 grades of concrete are used.
How to Make Concrete?
Concrete is manufactured or mixed in proportions w.r.t. cement quantity. There are two types of concrete mixes, i.e. nominal mix and design mix. The nominal mix is used for normal construction works such as small residential buildings. The most popular nominal mix is in the proportion of 1:2:4.
Design mixed concrete are those for which mix proportions are finalized based on various lab tests on cylinder or cube for its compressive strength. This process is also called a mixed design. These tests are conducted to find a suitable mix based on locally available material to obtain the strength required as per structural design. A design mixed offers economy on use of ingredients.
Once suitable mix proportions are known, then its ingredients are mixed in the ratio as selected. Two methods are used for mixing, i.e. Hand Mixing or Machine Mixing.
Based on the quantity and quality required, a suitable method of mixing is selected. In the hand mixing, each ingredient is placed on a flat surface, and water is added and mixed with hand tools. In machine mixing, different types of machines are used. In this case, the ingredients are added in the required quantity to mix and produce fresh concrete.
Once it is mixed adequately it is transported to the casting location and poured into formworks. Various types of formworks are available as selected based on usage.
Poured concrete is allowed to set in formworks for a specified time based on the type of structural member to gain sufficient strength.
After removal of formwork, curing is done by various methods to make up the moisture loss due to evaporation. Hydration reaction requires moisture which is responsible for setting and strength gain. So, curing is generally continued for a minimum of 7 days after the removal of formwork.
Types of Concrete Construction
Concrete is generally used in two types of construction, i.e. plain concrete construction and reinforced concrete construction. In PCC, it is poured and cast without the use of any reinforcement. This is used when the structural member is subjected only to the compressive forces and not bending.
When a structural member is subjected to bending, reinforcements are required to withstand tension forces, structural member, as it is very weak in tension compared to compression. Generally, the strength of concrete in tension is only 10% of its strength in compression.
It is used as a construction material for almost all types of structures such as residential concrete buildings, industrial structures, dams, roads, tunnels, multi-story buildings, skyscrapers, bridges, sidewalks, and superhighways, etc.
Examples of famous and large structures made with concrete are Hoover Dam, Panama Canal, and Roman Pantheon. It is the largest human-made building material used for construction.
Steps of Concrete Construction
The construction steps are:
- Selecting quantities of materials for selected mix proportion
- Mixing
- Checking of workability
- Transportation
- Pouring in formwork for casting
- Vibrating for proper compaction
- Removal of formwork after a suitable time
- Curing members with suitable methods and required time.
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